The kind of resentment that occurs when a wife continues to volunteer (volunteer, didn't realize that and now more than ever I encourage you to say that is a non-negotiable boundary) is much more damaging and severe that any resentment Mrs. Coconut could ever feel towards her husband who required her to discontinue such destructive and hurtful volunteering.
Besides ---> who should she really be upset or resentful at? She's the one that ruined this situation with her "mistake". Quiting that volunteer position is a consequence of her choices.
Of course, you are not her keeper. She's absolutely free to keep the volunteer position if she wants. You don't need to be angry or demanding. Just insist calmly that if she wants to maintain, recover and rebuild her relationship with you, the volunteer position must, for now (maybe express a minim of 1 year) go bye-bye.
Think about this. Compare her short lived little "mistake" affair (which I'm minimizing sardonically - because I think it's much more than a "mistake") coupled with her CONTINUING, indefinitely, to volunteer with that same guy and the resulting lifetime potential resentment quotient thereof to the lifetime potential resentment quotient of having to give up a volunteer firefighting position because she made out and carried on with a co-voluneer. They just don't compare in the least. The first one is the kind that results in divorce. The latter is merely a consequences of her choices and she'll get over it. Absent a great recovery, Cocunut is never going to get over his resentment and if forced to swallow her continuing hurtful to continue volunteering with OM, their marriage won't make it.
If I've said this once, I've said it a thousand times ----> YOU MATTER.
Coconut matters. She doesn't have to quit the job or be resentful about leaving it at all. She'd just be choosing that volunteer job over Coconut and then Coconut will know it's time for him to move on to someone that is actually considerate of his feelings. This isn't a test, but it's sure telling.
Waywards are full of selfishness and entitlement. I'd fully expect her to be angry and manipulative. To try to turn this into you being controlling or a jerk or even a dream crusher. You are nothing of the sort. She ruined the volunteer endeavor herself and someday, when the two of you have pieced together again an extraordinary marriage she'll see how right you were to require it. She may threaten back - threaten to not recover and leave you if you make her do this (which is her manipulative attempt to control you - because having that volunteer position is in no way a boundary for her to remain in your marriage). You must remain strong and await her coming back around to doing what she has to do.
Coming back to it being a volunteer position - volunteering does make a difference. This isn't her career. It's not putting food on the table. She's not missing out on a job she went to college for. She's only out a couple of months of part time "academy". MAYBE she can do it again in the future especially if OM moves on or leaves town OR you guys move. Running around all hours of the night with a bunch of men. Building such comradarie with a group of men all in the process of "getting off" being pseudo-firefighters (and the likely attitude they get firefighter lifestyle with it) is just way too much for you to have to endure. You are the victim here and shouldn't be made to feel guilty for requiring her behave trustworthy.
The internet is 90% complaining and entitlement and I hate it because I deserve better!